1. Field of the Invention
The field of this invention lies within the wire bonding art. It particularly lies within the art of ultrasonic wire bonding as it relates to ultrasonically bonding a wire on such items as electronic circuits and components. The invention is more specifically directed to testing wire bonds through a pull testing system. The pull testing system specifically integrates the process of the pull test with the process of wire bonding.
2. Description of the Prior Art
Prior to this invention the art of pull testing wire bonds was generally done with a pulling device or hook positioned underneath the loop of the bonded wire.
The pulling device or hook lifts up against the wire and places it in tension. This can be seen in FIG. 1 which shows an example of the prior art. The equipment is generally of a type that is stand alone equipment that either functions manually or automatically.
One method of pull testing is by means of a force being applied to the bonded wire loop by a motor driven loading arm. The hook or arm, which is connected to the free end of a flexible cantilever provides a certain force and a strain gauge on the beam measures the deflection. In this manner, the applied pulling force can be indicated by a signal corresponding to the measured force. Further to this extent, the system signifies the distance the hook moves when the strain gauge starts to generate its signal.
Other bonding tests incorporate certain devices to measure the bond's integrity by applying a known and infinitely variable tensile force to the wire under test.
Pull testing when used as a quality check for a bond is generally done in a non-destructive manner. A force is used to stress the bond so as to not cause damage to the wire or the bond. Further pull testing in the prior art can also be done in a destructive manner where the bond is pulled to its failure point. This method is only used on a sample basis since it destroys the bond it is measuring.
The pull force used to non-destructively pull test bonds uses a conventional pulling hook and is limited to a relatively low force. A high force can not be used inasmuch as the hook which is pulling across the axis of the length of the wire imparts a permanent kink, bend, bow or set in the wire. In other words, it bows the wire up, and distorts it from its original relationship as it overlies the circuit, or substrate.
A practical limit to non-destructive pulling forces is within ten to fifteen percent of the yield strength of the wire.
It has been thought that in order to eliminate the bow, the kink, or the angular upward orientation after a pull test is effectuated, the wire should be pulled along its axis. In such a manner, the pull test pulls the wire to a higher level of forces without damaging the wire. This invention enables this pull force along the length of the wire as opposed to across the wire.
Certain patents incorporate devices for testing wire bonds in the entire unit or bond head that moves to and from the bond to be tested. A gauge is incorporated which uses wire grasping hooks operatively connected to the gauge. After grasping a wire in a bonded configuration of a circuit, the gauge is moved away from the support until the wire bond fails and the force is registered by the gauge. In such destructive testing, as well as tests which limit destruction of the bond, substantial complexity of equipment is encountered.
The utilization of pulling hooks is limited in many conditions. The hooks are usually round and have a diameter that is two to three times the diameter of the wire that is to be tested. The hooks can also have a 90.degree. bend forming them into an L shape. These characteristics of the hooks limit them for use on wires that have a very low loop or are not substantially standing away from the surface between which the hooks must engage the wire. Furthermore, the hooks can not be used on wires that are closely bonded together, nor on wires that cross underneath other wires.
The state of the art for pull testing in the semiconductor and the hybrid industries is to use the pull testing as a quality check for the bonding process. It is generally a separate process that is performed after the bonding process is completed. In most cases, there is a significant time span between the bonding and the pull testing. This is not desirable, because in the time it takes to get results from the pull testing, several more rejected parts could be produced by the bonder. Thus, it is desirable to have a system that checks the quality of the bonding as soon as possible after making the bond.
Other methods of checking the quality of wire bonds is to monitor other variables during the bonding process such as the ultrasonic impedance, bond deformation, or frequency shifts. These second order monitoring systems have proven to be valuable in detecting some process variations, but are not as reliable as actually testing the bond through a pull test.
Some ultrasonic wire bonders incorporate a measure of bond quality obtained through non-destructive testing of the bond by developing a voltage proportional to the amplitude of the transverse motion of the ultrasonic bonding tool. The methods develop, by means of a transducer, a second voltage proportional to the tangential component of the forces applied during bonding. The voltages are then fed into a logic circuit to derive their ratio which is a measure of the bonding quality. These methods are not always reliable.
Further bond quality monitoring systems incorporate an analysis of the relative quality of bonding by means of the transducer power signal. In such cases, a series of analog computations based upon the logarithm of the impedance of the transducer is used in order to create a measure of bond quality.
Still other testers incorporate the utilization of a plurality of frequency readings that are taken and measured during the formation of the ultrasonic bond. The information is employed to calculate a bond quality rating, and predicting whether a good bond has been made. This is done by means of providing a signal indicative of a predetermined plurality of cycles from an ultrasonic generator in a stable source of pulses applied to a second counter. Here again, such methods are not deemed to be reliable.
A substantial object of this invention is to overcome the deficiencies of the foregoing prior art.
Another object of this invention is to provide a method and apparatus for pull testing wires that are closely oriented.
A further object is to provide a method of tensile stressing the wire and bond without degrading the bond.
Another object is to provide a fast and inexpensive way to test wire bonds.
Still further, the method and apparatus provides a method of stopping the wire bond process immediately after an inferior bond has been made.
Another object provides a method of testing wire bonds that eventually might become inaccessible by further assembly processes.
Still yet another object is to provide apparatus and a process for testing bonds that have a low loop of wire from the surface that would be incapable of normal conventional bond testing.
A further object is to provide a method of testing bonds that is not influenced by the angle of the wire forming the loop between the bonds.
Yet another object of this invention is to provide a method of pull testing a first bond independent of a second bond.